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Science: Molecular footprints in the sand

By JOHN EMSLEY

Impressions left by molecules in the surface of silica gel may act as
a catalyst for other similar molecules, but not for their mirror images,
according to a group of Japanese chemists. This is the first time anyone
has demonstrated that such a simple material can distinguish between the
left – and right-handed forms of a molecule, an ability which is essential
to the functioning of enzymes.

Kensaku Morihara and her colleagues at the Nara Women’s University in
Japan carried out their experiment with silica gel, a form of silicon dioxide
which has a surface covered with many cavities. Although silica is usually
chemically inert – it is chemically identical to ordinary sand – Morihara
says that, by retaining the imprint of a molecule which has been resting
on its surface, it is able to deter-mine the course of a reaction (Journal
of the Chemical Society Chemical Communications, 1992, p 358).

In their experiments, Morihara and her colleagues used a derivative
of the amino acid alanine, which has mirror-image forms referred to as L
and D. The chemists converted L-alanine to its amide by adding two benzoyl
groups, and they used this molecule to stamp a footprint on the surface
of the silica gel.

The chemists activated the silica gel with strong acid before treating
it with an aluminium salt and the alanine derivative. They left the solution
to ‘age’, so that both the alanine and aluminium had time to become attached
to the silica surface. Morihara believes that the aluminium ions bond to
the silica, then grip the oxygen atoms in the alanines.

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Once the aluminium ions were in place, the chemists removed the template
molecules by filtering off the silica gel and washing it with methanol.
This left an empty site on the surface of the silica which a similar molecule
could fit into rather like the last piece of a jigsaw puzzle.

To test the effect of imprinting the surface with the L-alanine benzoyl
amide, Morihara and her colleagues made an anhydride derivative of L-alanine
and reacted it with 2,4-dinitrophenol, a molecule which can attach itself
to the alanine in the presence of the silica gel. They found that the imprinted
silica gel was 10 times as effective as ordinary silica gel at catalysing
a chemical reaction.

Morihara says that ideally she would have liked to tried the same reaction
with D-alanine anhydride. She suspected that while the L anhydride fits
into the footprint made in the silica gel by the L amide, the same anhydride
of D-alanine, its mirror-image counterpart, would only partly fit the molecular
footprint.

Unfortunately, pure D anhydride proved impossible to make. However,
she was able to make a half-and-half mixture of the L and D forms, and found
that the pure L form of the alanine reacted much more rapidly than the mixture.
She says the L form acts three times as rapidly as the d form.

The results may explain how primitive life could have developed a preference
for one optical isomer of a molecule over its mirror image. On the beaches
of the primordial sea, it might have been possible for inorganic materials
such as sand to set evolution along the left-hand side of the road.